Updating postname

For exmaple:

When I click on the link ‘contact us’ the url looks like this:

http://www.sitename.com/contact-us

This I like…

However, when I click on the link ‘about us’ it looks like this:

http://www.sitename.com/sample-page

I want it to look like this:

http://www.sitename.com/about-us

Hopefully that made my question more clear.

Thanks!